The present disclosure relates generally to optical sensors, and more particularly, to backside illumination optical sensors in stacked assembly.
Optical sensors are electronic detectors that convert light into an electronic signal. In photography, a shutter is a device that allows light to pass for a determined period of time, exposing the optical sensors to the light in order to capture an image of a scene. Rolling shutter is a method of image capture in which a still picture or each frame of a video is captured by scanning across the scene rapidly in a horizontal or vertical direction. That is, every pixel is not captured at the same time; pixels from different rows are captured at different times. Rolling shutter is mostly used in cell phone sensors. Machine vision, in contrast, uses global shutter where every pixel is captured at the same time.
Most optical sensors use backside illumination. A back-illuminated sensor is a type of digital optical sensor that uses a particular arrangement of imaging elements to increase the amount of light captured, improving low-light performance. A traditional front-illuminated digital camera is constructed similar to the human eye, with a lens at the front and photodetectors at the back. This orientation of the sensor places the active matrix of the digital camera sensor, a matrix of individual picture elements, on its front surface and simplifies manufacturing. The matrix and its wiring, however, reflect some of the light, reducing the signal that is available to be captured. A back-illuminated sensor contains the same elements, but arranges the wiring behind the photocathode layer by flipping the silicon wafer during manufacturing and then thinning its reverse side so that light can hit the photocathode layer without passing through the wiring layer, thereby improving the chance of an input photon being captured.
However, conventional back-illuminated sensors tends to have higher leakage when exposed to light. Also, the photodiode fill factor, or the ratio of light-sensitive area of a pixel to total pixel area, is relatively low. A large fill factor is beneficial because more of the pixel area is used for photocollection, which simultaneously improves signal-to-noise ratio (SNR) and dynamic range. The dynamic range of an image sensor measures how wide of a range of lighting the sensor can accurately capture. The wider the dynamic range of the image sensor, the more details can be shown under low light conditions and thus the more versatile the imaging system becomes. The SNR of an image sensor measures the ratio between the signal and its associated noise. An image sensor with low SNR will have a high amount of noise appearing in the captured image. An image sensor with high SNR can be used in low light conditions.